The transmission of soliton pulses or "solitons" in the portion of an optical fiber that has abnormal dispersion is a known phenomenon. Solitons are pulse signals having a sech.sup.2 waveform. With pulses of this form, the non-linearity in the corresponding portion of the fiber compensates dispersion of the optical signal. Soliton transmission is modelled in known manner by the non-linear Schordinger equation.
Various effects limit the transmission of such pulses, such as the jitter induced by the solitons interacting with the noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus published in Optical Letters, Vol. 11, No. 10, pp. 665-667. This effect which is known as the "Gordon-Haus effect" or as "Gordon-Haus jitter" puts a theoretical limit on the quality or the bit rate of transmission by means of solitons.
To be able to overcome that limit, it is possible to use synchronous modulation of soliton signals with the help of semiconductor modulators. That technique intrinsically limits the bit rate of the soliton link because of the upper limit on the passband of a semiconductor modulator. Proposals have also been made for systems using sliding guiding filters that make it possible to control the jitter of transmitted solitons, see for example EP-A-0 576 208. Proposals have also been made, for the purpose of regenerating the line signal, to use the Kerr effect in synchronous amplitude or phase modulators. Finally, proposals have been made to regenerate soliton signals by using saturable absorbers.
Another proposal for increasing the bit rate of optical fiber transmission systems using soliton signals is to use wavelength division multiplexing (WDM). Under such circumstances, it is considered advantageous to use sliding guiding filters of the Fabry-Perot type, which filters are entirely compatible with wavelength division multiplexed signals. In contrast, the use of synchronous modulators or saturable absorbers for regenerating wavelength division multiplexed soliton signals is problematic because of the difference in group speeds between the signals in the various channels.
An article by E. Desurvire, O. Leclerc, and O. Audouin, published in Optics Letters, Vol. 21, No. 14, pp. 1026-1028, describes a wavelength allocation scheme which is compatible with the use of synchronous modulators. That article proposes allocating wavelengths to the various channels of the multiplex in such a manner that, for given intervals Z.sub.R between the repeaters, the signals on the various channels, or more exactly the bit times of the various channels in the multiplex, are substantially synchronized on arriving at the repeaters. This makes in-line synchronous modulation of all of the channels possible at given intervals with the help of discrete synchronous modulators. That technique of allocating multiplexed wavelengths is also described in French patent application 96/00732 file on Jan. 23, 1996 in the name of Alcatel Submarine Networks.
Another article by O. Leclerc, E. Desurvire, and O. Audouin, published in Optical Fiber Technology, 3, pp. 97-116 (1997) specifies that in such a wavelength allocation scheme, the bit times of subsets of the channels in the multiplex are synchronous at intervals that are submultiples of Z.sub.R. That article consequently proposes regenerating subsets of the channels in the multiplex at shorter intervals. Those solutions make it possible to transmit solitons over long distances, e.g. over transoceanic distances.
Soliton signal transmission systems are particularly adapted to high data rate transmission over long distances; one of the applications of such transmission systems is thus transoceanic transmission where typical lengths are a few thousands of kilometers.
One of the problems of a transoceanic transmission system is physical breakage of the transmission system or damage in the transmission system that cannot be repaired remotely. In conventional manner, under such circumstances, the undersea cable is raised and the damaged section is replaced. That solution is common practice and it is shown by way of example in FIGS. 1 and 2. As shown in FIG. 1, the cable 1 lies on the ocean bottom and has a damaged section 2. This section can be damaged in any way that is unsuitable for being repaired remotely. Under such circumstances, as shown in FIG. 2, the cable is raised and the damaged section is replaced by means of a length L of replacement cable 3.